CDMA is a wireless telecommunications technology in which basestations communicate with mobile units over a common bandwidth using quasi-orthogonal spreading codes to define individual communications channels. Such systems utilize "forward" and "reverse" traffic channels. A "forward" traffic channel carries traffic from a basestation to a mobile unit. A "reverse" channel carries traffic from the mobile unit to a basestation.
In a CDMA system, because different channels transmitting in the same direction share the same bandwidth, the communications signals on the various channels interfere with each other and contribute to an overall system interference level. CDMA system capacity, or the number of active mobile units that can be supported within a given system bandwidth, is limited by acceptable system interference level.
Further, in cellular telecommunications systems such as CDMA, as a mobile unit travels from one cell to another, service between the mobile unit and a given basestation (or basestations) must be transferred to one or more "new" basestations. This transfer process is termed "handoff." Handoff is normally triggered by some threshold mechanism related to received signal strength, call quality, system loading, or other system parameters.
Handoffs can be "hard" or "soft". A "hard" handoff is characterized by substantially simultaneous termination of communications with the "old" basestation and initiation of communications with the "new" basestation. The mobile unit is in communication with only one basestation at a time.
In a conventional "soft" or "make-before-break" handoff, the mobile unit and one or more "new" basestations establish communications with each other before communications with the "old" basestation (or basestations) are interrupted. Soft handoffs provide the advantage of "space diversity"--utilization of signals from geographically distributed locations during handoff--thereby improving signal coverage and quality.
However, during a soft handoff, since the mobile unit exchanges signals with multiple basestations, the handoff requires the allocation of resources from multiple basestations. The provision of multiple signals also generates additional interference for other links.
Thus, the advantages of soft handoff are attained at the expense of increased backhaul requirements, greater bandwidth (for the air interface) and processor utilization. With high speed or wideband data such as video, the "cost" of multiple links becomes much more significant than with voice.
By way of example, the IS-95 Standard promulgated by the Telecommunications Industry Association and the Electronic Industries Association (TIA/EIA/IS-95) sets forth parameters for operation of conventional CDMA cellular telecommunications systems. As a design objective, the IS-95 specifies a 2-way soft handoff zone at 30% of the service area, and a 3-way soft handoff zone at 10% of the service area. This requires allocation of 40% of system resources (radios, processors and other channel elements) to handoff, which could be otherwise used to support more traffic.
Accordingly, there exists a need for methods and systems that optimize bandwidth and processor usage during CDMA soft handoff.
It is therefore an object of the present invention to provide methods and apparatus that optimize bandwidth and processor usage in CDMA systems during soft handoff.
It is another object of the invention to provide such methods and apparatus that reduce the impact of power fluctuations caused by entry and exit of wideband users during soft handoff.
It is a further object of the invention to provide such methods and apparatus that reduce interference to other users during soft handoff.
These and other objects of the invention will become apparent to those skilled in the art from the following description.